High speed robots for small scale assembly utilize an end effector which places components in very precise locations. Typically, high precision end effectors must be very stiff in order to be repeatable. Any compliance in the design generally leads to a less repeatable mechanism. However, should a robot with a stiff end effector accidentally crash into a stationary object, the deceleration shock is transferred directly to the robot's transmission, and usually results in costly damage and significant downtime.
Several end effectors have been designed to withstand low speed crashes with high repeatability. One prior art design, Robotic Accessories' Uni-Coupler Safety Joint, model 4419, is capable of deflecting as much as 15 degrees from the axis of the end effector. This limitation renders that design useless for high speed crash protection. Although this model also includes an electrical switch that will cut power to the robot during a high speed crash, 15 degrees of deflection is insufficient to protect the robot. Moreover, the end effector can return to within only 0.002 inches of its original position after it is struck. This level of precision is inadequate for many applications such as head gimbal assembly pick-and-place tasks.
Another prior art device, described in U.S. Pat. No. 4,557,661, has a pair of arms which are connected to a boom via "shear pins." If the arms encounter a force greater than some predetermined design load, the shear pins will break to allow the arms to separate from the boom before the robot sustains any significant damage. Like the previous design, an electrical kill switch is used for additional protection. In this case, it would take additional time to recover the end effector from a crash since it would have to be recalibrated and the shear pins would have to be replaced. Moreover, in any crash, a significant shock would have to be transferred to the robot before the shear pins actually broke. Finally, because the mechanical properties of the shear pins cannot be precisely controlled, it is not possible to accurately define a "predetermined failure load." An end effector that can survive a high speed crash along any trajectory while limiting the shock transmitted to the robot and maintaining repeatability is needed.